The present invention relates to a process for manufacturing an antiperspirant stick.
It is generally known that a solid antiperspirant stick may be fabricated by suspending an antiperspirant salt in a liquefied carrier vehicle, pouring the resulting liquid suspension into a stick mold, and cooling to solidify the product. In such a process the carrier vehicle must be heated to a temperature sufficiently high to melt all of its components. Thus, where the carrier vehicle contains only relatively low melting components (m.p. &lt;70.degree. C.), it is not necessary to heat the batch above 70.degree. C. Typical of such systems are those described in U.S. Pat. No. 4,126,679 (Davy) and U.S. Pat. No. 4,722,836 (Geary). Both Davy and Geary melt a carrier vehicle comprising cyclomethicone and stearyl alcohol at about 65.degree. or 66.degree. C., add an ultrafine aluminum chlorohydrate salt, cool the mixture to a few degrees above the solidification point, pour the mixture into stick molds, and cool to solidify the product.
Where the carrier vehicle contains one or more high melting components (m.p. &gt;70.degree. C.), then the batch must be heated above 70.degree. C. in order to melt all the components. Typical of such systems are those described in U.S. Pat. No. 4,049,792 (Elsnau), U.S. Pat. No. 4,724,139 (Palinczar), U.S. Pat. No. 4,822,603 (Farris, U.S. Pat. No. 4,863,721 (Beck) and U.S. Pat. No. 4,985,238 (Tanner), which employ a high melting point wax (for example, castor wax) as one component of the carrier vehicle in addition to a variety of other typical low melting components (for example, volatile silicone, non-volatile silicone, stearyl alcohol, paraffinic hydrocarbons, PPG-14-butyl ether, among others). To manufacture an antiperspirant stick in accordance with the teachings of the above patents, the components of the carrier vehicle are mixed and heated to a temperature above 70.degree. C. sufficient to melt all of the components (typically above 80.degree. C.), the antiperspirant salt is added before the heating step or shortly after all the components are melted (that is, while the carrier vehicle is still hot), the mixture is cooled to a few degrees above the solidification point (typically 50.degree.-55.degree. C.), poured into stick molds, then cooled to solidify the product. A similar process is described in EP 396,137 (Vu), but the temperature is not allowed to exceed 80.degree. C. in order to prevent the polyethylene-vinyl acetate gelling agent from becoming cloudy.
It has now been discovered that when a particulate antiperspirant salt, especially one where 90% of the particles are below 11 microns, is suspended in a substantially anhydrous carrier vehicle which comprises at least one component which melts at 70.degree. C. or higher, and the suspension is heated above 70.degree. C. in a closed mixing vessel, then cooled in the conventional manner, the resulting antiperspirant stick contains a significant amount of agglomerated antiperspirant salt particles, typically over 60 microns in size, and in some cases in excess of 100 microns. The presence of such large particles significantly detracts from the aesthetics of the product, producing a gritty feel. Moreover, if the product contains no or inadequate suspending agents, such large particles will collect at the top or bottom of the container, depending on how it is filled, thus resulting in a non-homogeneous product. The presence of brownish crystalline particles substantially detracts from the appearance of the product as well.
As a result of considerable investigation and experimentation, and without being bound by any theory, it is believed that the afore-described process produces agglomerated antiperspirant salt particles by the following mechanism. When the temperature of the batch (that is, antiperspirant salt and carrier) exceeds 70.degree. C., and especially 80.degree. C., some of the water of hydration normally present in the antiperspirant salt is driven off, condenses on the mixing vessel walls and cover, then falls back into the solution as small droplets due to the agitation. These small droplets attract and agglomerate small antiperspirant salt particles, forming larger particles. Since the water which condenses on the mixing vessel is acidic, it tends to corrode the mixing vessel and carry back minute amounts of metal contaminants with it into the vehicle. These contaminants cause the agglomerated particles to become off-colored, typically brownish in color.
It is an object of the present invention to provide a process for producing a solid antiperspirant stick which contains a high melting component while substantially preventing the agglomeration of the antiperspirant salt into large particles.